If you’ve ever work in tech support, you’ve undoubtedly had users tell you “the Internet is down”. As a techie, you’re pretty sure that no one pulled the power cord from the Internet. Something must be going wrong somewhere between the user’s chair and the Internet.

Assuming its a technical problem, you begin to search for the problem. Perhaps the user’s computer is turned off, maybe their network cable is unplugged, or perhaps your organization’s core router just took a dive. Whatever the problem might be, one thing is most certain - the Internet isn’t down. It just happens to be unreachable for that user.

Shinken is able to determine whether the hosts you’re monitoring are in a DOWN or UNREACHABLE state. These are very different (although related) states and can help you quickly determine the root cause of network problems. To achieve this goal you must first and foremost define a check_command for the host you are monitoring. From there, here’s how the reachability logic works to distinguish between these two states...

Take a look at the simple network diagram below. For this example, let us assume you’re monitoring all the hosts (server, routers, switches, etc) that are pictured, meaning you have defined check_commands for each of the various hosts. Shinken is installed and running on the Shinken host.

If you have not defined a check_command for your host, Shinken will assume that the host is always UP. Meaning that the logic described will NOT kick-in.

In order for Shinken to be able to distinguish between DOWN and UNREACHABLE states for the hosts that are being monitored, you’ll need to tell Shinken how those hosts are connected to each other - from the standpoint of the Shinken daemon. To do this, trace the path that a data packet would take from the Shinken daemon to each individual host. Each switch, router, and server the packet encounters or passes through is considered a “hop” and will require that you define a parent/child host relationship in Shinken. Here’s what the host parent/child relationships look like from the viewpoint of Shinken:

Now that you know what the parent/child relationships look like for hosts that are being monitored, how do you configure Shinken to reflect them? The parents directive in your host definitions allows you to do this. Here’s what the (abbreviated) host definitions with parent/child relationships would look like for this example:

Now that you’re configured Shinken with the proper parent/child relationships for your hosts, let’s see what happen when problems arise. Assume that two hosts - Web and Router1 - go offline...

When hosts change state (i.e. from UP to DOWN), the host reachability logic in Shinken kicks in. The reachability logic will initiate parallel checks of the parents and children of whatever hosts change state. This allows Shinken to quickly determine the current status of your network infrastructure when changes occur.

In this example, Shinken will determine that Web and Router1 are both in DOWN states because the “path” to those hosts is not being blocked.

Shinken will determine that all the hosts “beneath” Router1 are all in an UNREACHABLE state because Shinken can’t reach them. Router1 is DOWN and is blocking the path to those other hosts. Those hosts might be running fine, or they might be offline - Shinken doesn’t know because it can’t reach them. Hence Shinken considers them to be UNREACHABLE instead of DOWN.

By default, Shinken will notify contacts about both DOWN and UNREACHABLE host states. As an admin/tech, you might not want to get notifications about hosts that are UNREACHABLE. You know your network structure, and if Shinken notifies you that your router/firewall is down, you know that everything behind it is unreachable.

If you want to spare yourself from a flood of UNREACHABLE notifications during network outages, you can exclude the unreachable (u) option from the “notification_options” directive in your host definitions and/or the “host_notification_options” directive in your contact definitions.